The 3GPP (3rd Generation Partnership Project) which is an international mobile communication standardization group has started the standardization of LTE-Advanced (Long Term Evolution-Advanced, LTE-A) as a fourth generation mobile communication system. As in Non-patent Literature 1, in LTE-A, a relay technology of relaying radio signals by using a relay node is being studied with the goals of coverage expansion and capacity improvement.
Now, referring to FIG. 20, the relay technology will be described. FIG. 20 is a diagram showing a wireless communication system that relays radio signals by using the relay technology. In FIG. 20, eNB represents a base station, RN represents a relay node, and UE represents a wireless communication terminal. Moreover, UE1 represents a wireless communication terminal connected to eNB, and UE2 represents a wireless communication terminal connected to RN.
In LTE-A, it is being studied that RN has an individual cell ID as in eNB, and thereby, when viewed from UE, RN can be regarded as one cell like eNB. eNB is connected to a network by wired communication, whereas RN is connected to eNB by wireless communication. A communication channel connecting between RN and eNB is called a backhaul channel. On the other hand, a communication channel connecting between eNB or RN and UE is called an access channel.
On the downlink channel, for example as shown in FIG. 20, RN receives signals from eNB on the backhaul channel (an arrow A in the figure), and transmits the signals to UE2 on the access channel of RN (an arrow B in the figure). When the backhaul channel and the access channel are allocated in the same frequency bandwidth, if RN performs transmission and reception at the same time, an interference due to feedback occurs. For this reason, RN cannot perform transmission and reception at the same time. Consequently, in LTE-A, a relay method is being studied in which the backhaul channel and the access channel of RN are allocated while being divided by the time domain (in units of subframes).
Referring to FIG. 21, the above-mentioned relay method will be described. FIG. 21 is a diagram showing the subframe structure on the downlink channel in the relay method. Reference designations [n, n+1, . . . ] in the figure represent subframe numbers, and boxes in the figure represent subframes on the downlink channel. Moreover, the following are shown: transmission subframes of eNB (crosshatched parts in the figure), reception subframes of UE1 (blank parts in the figure), transmission subframes of RN (rightward hatched parts in the figure), and reception subframes of UE2 (leftward hatched parts in the figure).
As shown by the arrows (thick lines) in FIG. 21, signals are transmitted from eNB in all the subframes [n, n+1, . . . , n+6]. Moreover, as shown by the arrows (thick lines) or the arrows (broken lines) in FIG. 21, UE1 is capable of performing reception in all the subframes. On the other hand, as shown by the arrows (broken lines) or the arrows (thin lines) in FIG. 21, at RN, signals are transmitted in the subframes except for the subframe numbers [n+2, n+6]. Moreover, as shown by the arrows (thin lines) of FIG. 21, UE2 is capable of receiving signals in the subframes except for the subframe numbers [n+2, n+6]. And RN receives signals from eNB in the subframes of the subframe numbers [n+2, n+6]. That is, at RN, the subframes of the subframe numbers [n+2, n+6] serve as the backhaul channel, and the other subframes serve as the access channel of RN.
However, if RN transmits no signal from eNB in the subframes [n+2, n+6] where RN serves as the backhaul, a problem arises in that a measurement operation to measure the quality of RN does not function at an LTE wireless communication terminal that does not know the presence of RN. As a method of solving this problem, in LTE-A, it is considered to use an MBSFN (Multicast/Broadcast over Single Frequency Network) subframe defined in LTE.
The MBSFN subframe is a subframe prepared to realize an MBMS (Multimedia Broadcast and Multicast Service) service in the future. The MBSFN subframe is designed to transmit cell-specific control information at the first two symbols and transmit signals for the MBMS in the domains of the third and subsequent symbols. Consequently, LTE wireless communication terminals are capable of performing measurement by using the first two symbols in the MBSFN subframe.
The MBSFN subframe can be spuriously used in RN cells. That is, in the RN cell, at the first two symbols of the MBSFN subframe, the control information specific to the RN cell is transmitted, and in the domains of the third and subsequent symbols, signals from eNB are received without the data for the MBMS being transmitted. Consequently, in RN cells, the MBSFN subframe can be used as the reception subframe on the backhaul channel. Hereinafter, the MBSFN subframe spuriously used in the RN cell as mentioned above will be called “MBSFN subframe that RN uses as the backhaul”.
Here, in the subframes [n+2, n+6] of RN in FIG. 21, since no signal is transmitted from RN, for UE1, the interference from RN is eliminated, so that SIR (signal to interference power ratio) improves. eNB positively allocates UE where SIR improves in the subframes [n+2, n+6], so that the user throughput at UE improves and this improves the throughput of the whole cells. Therefore, to improve the throughput of the whole cells, it is necessary for eNB to know the channel quality at UE.